Bright zinc plating from an acid electrolyte

ABSTRACT

MIRROR BRIGHT ZINC ELECTRODEPOSITS ARE FORMED FROM ACID ZINC PLATING SOLUTIONS CONTAINING AS BRIGHTENERS N-POLYVINYLPYRROLIDONE-(2) AND A KETONE OF THE FORMULA   R1-(CH=CH)N-CO-R2   WHEREIN R1 IS PHENYL, MONO- OR DIHYDROXYPHENYL OR TRIHYDROXYPHENYL METHYLENEDIOXYPHENYL, MONO- DI- OR TRILOWER-ALKYLPHENYL, MONO- OR DI- OR TRI-LOWER-ALKOXYPHENYL, FURYL, THIENYL, COUMARYL, OR PYRIDYL, AND R2 MAY BE LOWER ALKYL, LOWER ALKENYL, LOWER-ALKYL-CARBONYL-LOWERALKYLENE, CYANO-LOWER-ALKYLENE, LOWER-ALKOXY-CARBONYLLOWER-ALKYLENE, OR -(CH=CH)N-C6H5, N BEING 1 OR 0.

United States Patent ()lfice US. Cl. 204-55 8 Claims ABSTRACT OF THE DISCLOSURE Mirror bright zinc electrodeposits are formed from acid zinc plating solutions containing as brighteners N-polyvinylpyrrolidone-(Z) and a ketone of the formula wherein R is phenyl, monoor dihydroxyphenyl or trihydroxyphenyl methylenedioxypheny'l, mono-, dior trilower-alkylphenyl, monoor dior tri-lower-alkoxyphenyl, furyl, thienyl, coumaryl, or pyridyl, and R may be lower alkyl, lower alkenyl, lower-alkyl-carbonyl-loweralkylene, cyano-lower-alkylene, lower-alkoxy-carbonyllower-alkylene, or -(CH CH) C H n being 1 or 0.

This invention relates to the electrodeposition of bright zinc, and particularly to bright zinc plating from acid electrolytes.

A wide variety of bright zinc plating solutions of the cyanide type is available. The cyanide baths are relatively unstable because of the decomposition of cyanide to carbonate, and the cyanide drag-out into rinsing baths presents a serious disposal problem. The deposits formed from known acid zinc plating baths free from organic complexing agents are at best smooth, but not bright, and complex forming organic components in acid zinc plating solutions, such as EDTA present a disposal and pollution problem as difiicult to solve as that caused by cyanide. Zinc and other heavy metals cannot be precipitated from the rinse water and other wastes bearing EDTA by means of alkaline reagents. Bright coatings can be produced from baths containing EDTA (ethylenediaminetetra-acetic acid) only if other heavy metals, particularly iron, copper, and nickel, are carefully and initially removed.

The object of the invention is the provision of a brightener system for acid zinc plating solutions which permits the zinc dragged out into rinsing baths to be precipitated by an alkaline reagent because of the absence of complex forming compounds, is insensitive to substantial amounts of other heavy metal, provides bright and ductile deposits over a wide range of cathode current densities, and is stable under the operating conditions usual for the electrodeposition of dull coatings from acid electrolytes including agitation by means of compressed air.

Such a brightener system has been found in the combination of N-polyvinylpyrrolidone-( 2) with a secondary brightener of the formula wherein R is R is lower alkyl, lower alkenyl, lower-alkyl-carbonyllower-alkylene, cyano-lower-alkylene, lower-alkoxycarbonyl-lower-alkylene, or ---(CH CH) C H 3,594,291 Patented July 20, 1971 R R and R each are hydrogen, hydroxy, lower alkyl, or lower alkoxy, and R and R together represent the methylenedioxy group, and

n is 1 or 0.

The concentration of polyvinylpyrrolidone-(Z) in the electrolytes of the invention may be between 0.5 to g./l., and that of the secondary brightener between 0.01 and 0.5 g./l., with best results usually being obtained at respective concentrations of 2-l0 g./l. and 0.050.3 g./l. For reasons not fully understood at this time, the two brighteners synergistically cooperate, the brightness of the electrodeposit obtained by their simultaneous presence being far superior to the sum of their individual effects. Neither of them alone can produce the mirror brightness available from the combination.

The brighteners of the invention are known compounds, and may be prepared from readily available starting ma terials by known methods. The solution polymerization of vinyl pyrrolidone in the presence of azoisobutyronitrile has been described in German Pat. No. 922,378.

The degree of polymerization of the polyvinylpyrrolidone is not critical, acceptable results being obtained with products whose degree of polymerization is between 5 and 1000. Most consistently satisfactory results are achieved with a polymer having an average molecular weight of about 10,000 to 30,000, and a corresponding intrinsic viscosity of about 17 to 25.

Representative secondary brighteners of the invention II o Acetophenone ll Ethinylphenylketone Propiophenone II o Benzalacetone 25 OH 2-Hydroxybenzalacetone II 0 CH 3-Methoxybenzalacetone @Cli CH-C-CH;

g CH CH3 2,3-Dimethyl-benzalacetone CHzO Piperonylidcnebenzalacetone Benzoylacetone Benzoylacetonitrile ll C-CH3 3-Acetyl-coumarin CCH3 3-Acetylpridine 2,4-Dihydroxy-benzophenone Benzalacetophenone Dibenzalacetone Ethylbenzoylacetate Oou=on oon o t 1-(2-Furyl)-1'-buten-3-one Z-Thienylideneacetone OH 3,4,5-Trihydroxybenzalacetone r --CH=CH-h3 O 2-Methylbenzalacetone 3,4,S-Trimethylbenzalacetone (5 CH3 3,4-Dimethoxybenzalacetone The aqueous electrolyte which may be improved by the use of the brighteners may typically contain zinc sulfate and ammonium chloride, and optionally boric acid within the following ranges:

ZnSO -7H O -600 NH4CI 10-100 H3BO3 0-40 The zinc sulfate may be replaced as a source of zinc ions partly or entirely by other Zinc salts such as the chloride, acetate, fiuoborate, and sulfamate. Ammonium chloride has the primary function of improving the conductivity of the electrolyte, and may therefore be replaced by other conductive salts such as sodium chloride, sodium sulfate, sodium acetate, aluminum sulfate, and many others, as is well known in itself according to the art.

If so desired, other known brighteners may be employed in conjunction with those of this invention, known thiadiazol derivatives being particularly suited for this purpose. Wetting agents and other conventional adjuvants may be added to perform their known functions.

The electrolytes of the invention may be operated over a wide range of plating conditions, particularly a pH value of 3 to 6, a temperature of 10 to 45 0, most conveniently at 20-30 C., and at cathode current densities from 0.1 to 10.0 amps. per dm For highest brightness, cathode rod agitation or aeration is necessary. Still baths produce coatings which are not truly mirror bright, particularly at relatively high current densities. The electrolytes of the invention are particularly useful for barrel plating, both in oblique barrels containing the electrolyte and in horizontal barrels immersed in a tank. Racked objects have been handled successfully on conveyors, with highest brightness being achieved with air agitation. Such zinc plates of the invention are completely free from haze.

The brighteners of the invention undergo neither chemical nor electrolytic decomposition in the acid zinc plating solutions and can be used for a long time without additions other than those needed to make up for drag-out losses. They are relatively insensitive to contamination with heavy metals other than zinc, particularly to iron contamination. The electrolytes of the invention are not impaired if steel parts are accidentally immersed in the bath without current as may occur if a rack drops into the electrolyte from a conveyor.

A few zinc plating solutions of the invention and the operating conditions under which they were operated successfully to produce fully bright zinc plates are listed in the following examples.

EXAMPLE 1 Zinc sulfate (ZnSO -7H O): g./l. Ammonium chloride: 53 g./l.

Boric acid: 20 g./l. Polyvinylpyrrolidone-(2): 3 g./l. Benzalacetone: 0.1 g./l.

Temperature: 25 C.

Current density: 0.1-5.0 amps./drn. Air agitation EXAMPLE 2 Zinc sulfate (ZnSO -7H O-): 290 g./l.

Ammonium chloride: 25 g./l.

Polyvinylpyrrolidone-(Z): 5 g./l.

Propiophenone: 0.1 g./l.

Nonylphenol-ethyleneoxyde addition compound:

Temperature: 30 C.

Current density: 0.1-60 amps/din.

Cathode rod agitation EXAMPLE 3 Zinc chloride (ZnCl 70 g./l. Ammonium sulfate: 130 g./l. Sodium chloride: g./l. Polyvinylpyrrolidone(2): 3 g./l. Benzalacetone: 0.1 g./l. Ethyl benzoylacetate: 0.1 g./l. Sodium ethylhexylsulfate: 0.5 g./l. pH: 5.0 Temperature: 25 C. Current density: 0.1-5.0 amps/drn. Air agitation EXAMPLE 4 Zinc sulfate (ZnS-O -7H O): 145 g./l. Ammonium chloride: 53 g./l. Boric acid: g./l. Polyvinylpyrrolidone-(Z): 3 g./l. 1-(2-Furyl)-1-buten-3-one: 0.1 g./l. Nonylphenol-ethylenoxide 0.4 g./l. pH: 4.2 Temperature: C. Current density: 0.1-5 amps/dm. Cathode rod agitation EXAMPLE 5 Zinc sulfate (ZnSO -7H O): 290 g./l. Ammonium chloride: 25 g./l. -Polyvinylpyrrolidone-(2): 5 g./l. S-Acetylpyridine: 3 g./l.

Fatty alcohol polyglycolether: 0.2 g./l. pH: 4.8

Temperature: 'C.

Current density: 0.1-7.5 A./dm. Cathode rod agitation EXAMPLE 6 Zinc sulfate (ZnSO -7H O): 145 g./l.

Ammonium chloride: 53 g./l. Polyvinylpyrrolidone-(Z): 2.5 g./l. 1-(2-Furyl)-1-buten-3-one: 0.2 g./1.

Sodium dodecylphenol-polyglycol-ether-sulfate: 0.1 g./l. pH: 5.0

Temperature: 25 C.

Current density: 0.1-5 amps/dm.

Cathode rod agitation EXAMPLE 7 Zincsulfate (ZnSO -7H O): 145 g./l. Ammoniumchloride: 53 g./l. Boric acid: 20 g./l. Polyvinylpyrrolidone-(2): 3 g./l. S-methoxy-benzaleacetone: 0.15 g./l. pH: 5.5 Temperature: 25 C. Current density: 0.1-5.0 A./dm. Air agitation EXAMPLE 8 Zinc sulfate (ZnSO -7H O): 290 g./l. Ammoniumchloride: 25 g./l.

6 Polyvinylpyrrolidone-(Z): 5 g./l. 4-methoxy-benzylideneaceton2 0.05 g./l. Nonylphenol-ethylenoxyde addition compound: 0.04 g./l. pH: 4.5 Temperature: 30 C. Current density: 0.1-6 amps/drn. Cathode rod agitation EXAMPLE 9 Zinc chloride (Z CI 70 g./l. Ammonium sulfate: g./l. Sodiumchloride: 10 g./l. Polyvinylpyrrolidone-(2): 3 g./l. Piperonylideneacetylaceton: 0.2 g./l. Sodium ethylhexylsulfate: 0.5 g./l. pH: 5.0 Temperature: 25 C. Current density: 0.1-5 amps/din. Air agitation EXAMPLE 10 Zinc sulfate (ZnSO -7H O): g./l. Ammoniumchloride: 53 g./l. Boric acid: 20 g/l. Polyvinylpyrrrolidone-(2): 3 g./l. Piperonylidenebenzyalaceton: 0.1 g./l. Nonylphenol-ethyleneoxide: 0.4 g./l. pH: 4.2 Temperature: 25 C. Current density: 0.1-5 amps/dm. Cathode rod agitation EXAMPLE 11 Zinc sulfate (ZnSO -7H O): 145 g./l. Ammoniumchloride: 53 g./l.

Polyvinylpyrrolidone-(2): 2.5 g./l. Z-thienylideneaceton: 0.2 g./1.

Sodium dodeeylphenol-polyglycolether-sulfate: 0.25 g./l. pH: 4.8

Temperature: 30 C.

Current density: 0.1-5 amps/dm.

Cathode rod agitation EXAMPLE 12' Zinc sulfate (ZnSO -7H O): 290 g./l. Ammoniumchloride: 25 g./l. Polyvinylpyrrolidone-(Z): 5 g./l. A-acetylcoumarin: 0.4 g./l.

Temperature: 30 C.

Current density: 0.15 amps/dm. Cathode rod agitation What is claimed is:

1. In an aqueous acid plating electrolyte for the deposition of zinc electrodeposits in the absence of organic compounds forming complex ions with heavy metals, the electrolyte containing a source of zinc ions, the improvement which comprises having present in said electrolyte:

(a) 0.5 to 100 grams per liter N-polyvinylpyrrolidone- (2) dispersed in said electrolyte as a primary brightener; and

(b) .01 to 0.5 gram per liter of a secondary brightener dispersed in said electrolyte, said secondary brightener being a compound of the formula R is lower alkyl, lower alkenyl, lower alkyl-carbonyllower-alkylene, cyano-lower alkylene, lower-alkoxycarbonyl-lower-alkylene, or

R R and R each are hydrogen, hydroxy, lower alkyl, or lower alkoxy, and R and R together represent the methylenedioxy group, and

n is 1 or 0.

2. In an electrolyte as set forth in claim 1, the concentration of said N-polyvinylpyrrolidone-(2) being between 2 and 10 grams per liter, and the concentration of said secondary brighteners between 0.05 and 0.3 gram per liter.

3. In an electrolyte as set forth in claim 1, R being methyl or ethyl.

4. In an electrolyte as set forth in claim 3, R being ethenyl.

5. In an electrolyte as set forth in claim 1, R being CH -CO-CH 6. In an electrolyte as set forth in claim 1, R being 7. In an electrolyte as set forth in claim 1, the pH being 3 to 6.

8. A method of forming bright zinc electrodeposits which comprises making an object the cathode in the electrolyte as set forth in claim 1.

References Cited UNITED STATES PATENTS 2,853,444 9/1958 Pye et a1. 204-55X 3,005,759 10/1961 Safranek et al 20455 3,285,840 11/1966 Lindemann 204-55 GERALD L. KAPLAN, Primary Examiner 

